1. Field of the Invention
The present invention relates to a driving circuit for driving a piezoelectric vibrator. More particularly, the present invention pertains to a single-transistor type driving circuit for driving a piezoelectric vibrator.
2. Description of the Prior Art
In Japanese patent application Sho No. 61-309113 which has been filed on Dec. 27, 1986 and disclosed for public inspection on July 11, 1988 under the disclosure No. 63-167098 there is disclosed an ultrasonic liquid pump which is similar in structure to a bolted Langevin type vibrator and can by itself pump and atomize the liquid. In this type of liquid pump, the liquid to be pumped constitute the load on the pump so that the load is changed in response to a change in the liquid level. Such load change results in a change in the resonating frequency of the vibrator. In order to drive the vibrator with a high efficiency, it is therefore necessary to control the frequency of the driving voltage applied to the driving circuit. In other types of vibrators, the resonating frequency will be changed depending on the load applied thereto. In the case of a piezoelectric vibrator, the resonance point will change even in response to a change in the driving voltage. It is therefore required in these vibrators to control the driving frequency in accordance with a change in the resonance point.
Hithertofore, several types of driving circuits are known for driving a piezoelectric vibrator. Such driving circuits include a two transistor type such as a push-pull type and a half bridge type as well as a single transistor type such as an A-class amplifier, an oscillator and a switching circuit. In order to control the driving frequency in response to a change in the resonance point of the vibrator, a proposal is made to use an equivalent impedance portion provided by the vibrator at a region between the resonating frequency and the non-resonating frequency as in a Colpitz oscillator. Alternatively, it is also proposed to control in accordance with the minimum impedance which appears at the resonance point of the vibrator. Examples of the alternative control are the one in which control is made so that the current through the vibrator is maximized and the one in which the current and the voltage in the vibrator are detected and a control is made so that the current and the voltage have the same phase.
Referring to FIG. 5, there is shown an example of a conventional two transistor type push-pull driving circuit which includes a pair of transistors Q2 and Q3 connected with a primary winding of an output transformer T2 in a push-pull relationship. The transformer T2 has a secondary winding which is connected with a piezoelectric vibrating element TD. The transistors Q2 and Q3 have bases which are applied with driving voltage of opposite phase. The primary winding of the transformer T2 has an intermediate terminal which is connected with a bus voltage VB.
FIG. 6 shows an example of a conventional half bridge type driving circuit which includes a pair of transistors Q4 and Q5 connected in series between terminals leading to the power source VB. Between the terminals from the power source VB, there are a pair of capacitors C1 and C2 which are connected in series. The output transformer T3 has a primary winding connected on one hand with a connection between the transistors Q4 and Q5 and on the other hand with a connection between the capacitors C1 and C2. The transformer T3 has a secondary winding which is connected with a piezoelectric vibrating element TD. The transistors Q4 and Q5 have bases which are applied with driving voltage of opposite phase.
It has been recognized that the driving circuits shown in FIGS. 5 and 6 are suitable for a piezoelectric vibrator having a large power consumption. It should however be noted that the circuit requires two transistors and two driving signals of opposite phase so that the arrangements are complicated as compared with a single transistor type circuit. Further, this type of circuit is disadvantageous in that a reverse electromotive power produced in the piezoelectric vibrator influences from the secondary winding to the primary winding of the transformer to prevent the transistor from being switched from the on state to the off state. This will have an adverse effect on an effort of improving the efficiency of the circuit. It should further be noted that in an arrangement wherein any fluctuation of the source voltage is compensated for through a control of the pulse width of the driving signal, the operation may become unstable due to the aforementioned reverse electromotive power.
Referring to FIG. 7, there is shown a conventional driving circuit of a single transistor type in which a transistor Q5 is connected with the source voltage VB in series with the primary winding of the output transformer T4. The secondary winding of the transformer is connected with a piezoelectric vibrator TD. The circuit shown in FIG. 7 is considered to be advantageous over the two transistor type in that the circuit arrangement is simple and the control can be readily carried out in response to a change in the source voltage by changing the pulse width of the driving signal. The circuit however is difficult for use with a vibrator of a large power consumption because the transistor will be subjected to a substantial load. Since there is no transistor which can absorb the reverse electromotive power produced in the vibrator TD, the collector of the transistor may be subjected to a voltage of a substantial value. Therefore, the transistor must be of a high voltage type.
The Colpitz oscillator is known as a type which utilizes for the driving circuit control an equivalent inductance which the vibrator provides at an intermediate region between the resonating frequency and the non-resonating frequency of the vibrator. The Colpitz type oscillator is widely used in an oscillating circuit and mostly uses a quartz oscillator. A piezoelectric element is similar to a quartz oscillator in many aspects, however, in a certain property, the former is different from the latter. More specifically, referring to FIG. 8 which shows an impedance change in response to a frequency change, it should be noted that there is a substantial difference between the resonance frequency fr and the non-resonance frequency far in the case of a piezoelectric element. For this reason, it is practically impossible to obtain a high stability.
It should further be noted that the single transistor type circuit applied in the manner similar to the Colpitz oscillator is not suitable for driving a ultrasonic pump having a structure similar to that of a bolted Langevin oscillator and adapted for pumping and atomizing liquid. Further, the circuit of this type is not suitable for an application to a ultrasonic machining apparatus or to a ultrasonic welding machine which requires a high electric power. The circuit of this type is designed to drive the vibrator at a frequency between the resonating frequency and the non or anti-resonance frequency so that the system cannot be operated under the resonating frequency under which a most efficient driving can be accomplished.
In a driving circuit of the type in which the control is carried out based of the minimum impedance at the resonance point of the vibrator, the circuit may include a transistor type switching device. In this type, however, difficulty of control is in practice encountered since voltage or current of a sinusoidal form is not applied to the piezoelectric element. It is therefore desirable to provide a driving circuit in which a high frequency output of a sinusoidal form is produced. It should however be noted that with an output of a sinusoidal form it is impossible to have the output transistor operated under a high efficiency.
A switching circuit may be provided in the driving circuit so that the efficiency of the output transistor can be increased. In this arrangement, however, the current and the voltage at the vibrator will be of distorted configurations so that it becomes practically impossible to control the current and the voltage so that they have the same phase. It may be considered to carry out the control so that the current at the piezoelectric element is maintained at a maximum value. However, this solution is not satisfactory because it is difficult to detect the maximum value of the current due to the distorted form of the current. Further, there is a possibility that the maximum value of the vibrating element changes from time to time and also depending on the load so that the control becomes further difficult.